Thrusting and Exhumation of the Southern Mongolian Plateau: Joint Thermochronological Constraints from the Langshan Mountains, Western Inner Mongolia, China

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Feng, L.-X., Brown, R. W., Han, B.-F., Wang, Z.-Z., Łuszczak, K., Liu, B., Zhang, Z.-C. and Ji, J.-Q. (2017) Thrusting and exhumation of the southern Mongolian Plateau: Joint thermochronological constraints from the Langshan Mountains, western Inner Mongolia, China. Journal of Asian Earth Sciences, 144, pp. 287-302. (doi:10.1016/j.jseaes.2017.01.001)

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Thrusting and exhumation of the southern Mongolian Plateau: joint thermochronological constraints from the Langshan Mountains, western Inner Mon- golia, China

Li-Xia Feng, Roderick W. Brown, Bao-Fu Han, Zeng-Zhen Wang, Katarzyna Łuszczak, Bo Liu, Zhi-Cheng Zhang, Jian-Qing Ji

PII: S1367-9120(17)30001-9 DOI: http://dx.doi.org/10.1016/j.jseaes.2017.01.001 Reference: JAES 2919

To appear in: Journal of Asian Earth Sciences

Received Date: 13 May 2016 Revised Date: 3 January 2017 Accepted Date: 4 January 2017

Please cite this article as: Feng, L-X., Brown, R.W., Han, B-F., Wang, Z-Z., Łuszczak, K., Liu, B., Zhang, Z-C., Ji, J-Q., Thrusting and exhumation of the southern Mongolian Plateau: joint thermochronological constraints from the Langshan Mountains, western Inner Mongolia, China, Journal of Asian Earth Sciences (2017), doi: http://dx.doi.org/ 10.1016/j.jseaes.2017.01.001

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Thrusting and exhumation of the southern Mongolian Plateau: joint thermochronological constraints from the Langshan Mountains, western Inner

Mongolia, China

Li-Xia Fenga, Roderick W. Brownb, Bao-Fu Hana*, Zeng-Zhen Wanga, Katarzyna

Łuszczakb,c, Bo Liua, Zhi-Cheng Zhanga, Jian-Qing Jia

a Key Laboratory of Orogenic Belts and Crustal Evolution of Ministry of Education,

School of Earth and Space Sciences, Peking University, Beijing 100871, China b School of Geographical and Earth Sciences, College of Science and Engineering,

University of Glasgow, Glasgow G12 8QQ, United Kingdom c Scottish Universities Environmental Research Centre, East Kilbride G75 0QF, United

Kingdom

Corresponding author: Bao-Fu Han

Email address: [email protected]

ABSTRACT

The Mongolian Plateau has undergone multi-stage denudation since the Late

Triassic, and the NE-trending Langshan Mountains in the southern margin of the

Mongolian Plateau is crucial to unraveling the Meso-Cenozoic cooling and exhumation history of the Mongolian Plateau. The Langshan Mountains are dominated by

Precambrian gneiss and Permian–Middle Triassic granitic plutons crosscut by a set of

NE-striking thrust faults. A joint thermochronological study was conducted on 31 granitic and gneissic samples along the HQ and CU transects across the Langshan

Mountains and other two samples from the BQ in the north of the Langshan

Mountains. Four biotite/muscovite and three K-feldspar 40Ar/39Ar plateau ages range from 205 ± 1 to 161 ± 1 and 167 ± 1 to 131 ± 1 Ma, respectively. Thirty three apatite fission track (AFT) ages are between 184 ± 11 and 79 ± 4 Ma, with mean track lengths from 11.1 ± 1.8 to 13.1 ± 1.4 μm of mostly unimodal distributions. Thirty one single-grain raw AHe ages are in a range of 134 ± 8 to 21 ± 1 Ma. The AFT ages decrease monotonously from NW to SE until thrust faults along the two transects, with an age-jump across thrust F35. Joint thermal history modelling shows a three-stage cooling history as a result of denudation, especially with spatial differentiation in the first stage. Relative slow cooling at c. 0.6–1.0 °C/Ma occurred in the BQ and the northern part of the HQ transect during 220–100 Ma and the northern part of the CU transect during 160–100 Ma, respectively, with an amount of c. 2–3 km denudation between 160 and 100 Ma, implying little movement along the thrusts F13 and F33. In the middle and southern parts of the HQ transect and the southern part of the CU transect, rapid cooling at c. 4.0–7.0 °C/Ma, with c. 6–9 km denudation during 170–130

or 160–100 Ma, respectively, was probably influenced by thrusting of F35, F38 and

F42 and the resultant tilting. A combination of thrusting, tilting, and denudation led to the youngering trends towards thrusts in different parts. However, there was no significant denudation across the Langshan Mountains in the second stage from

100–80 Ma until the last stage of rapid denudation (c. 2 km) since 20–10 Ma, which was simultaneous with the rapid uplift of the northern part of the Tibetan Plateau at c.

15 Ma. A youngering trend of AFT ages from the inner to the peripherals of the

Mongolian Plateau implies the outward propagation of the Mongolian Plateau since the

Mesozoic.

Key words

The Langshan Mountains; The Mongolian Plateau; Thrust; Exhumation;

Biotite/Muscovite and K-feldspar 40Ar/39Ar; Apatite fission track; Apatite (U-Th)/He;

Thermal history modelling.

1. Introduction

The Mongolian Plateau is within the Asain continent and has an average elevation of 2000 m above sea-level and is surrounded by the Baikal area, the Sayan Mountains, the Altay Mountains, the Gobi-Altay Mountains, the Beishan Mountains, the Langshan

Mountains, the Daqingshan Mountains, the Yanshan Mountains, and the Greater

Hinggan Mountains (Fig. 1). Geologically, the Mongolian Plateau comprises part of the

Central Asia Orogenic Belt (CAOB) and the northern margin of the North China Craton

(NCC), and they are separated by the Chifeng-Bayan Obo Fault (Fig. 1). The CAOB is a major Phanerozoic accretionary orogen seated between the European Craton (EC) to the west, the Siberian Craton (SC) to the east, and the NCC to the south (e.g., Windley et al., 2007; Xiao et al., 2009). The accretion process of the CAOB is related to the tectonic evolution of the Paleo-Asian Ocean (e.g., Tang et al., 1990; Han et al., 2011;

Xu et al., 2013) and the final closure of the Paleo-Asian Ocean resulted in the amalgamation of the NCC and CAOB during the Late Permian to Triassic (e.g., Xiao et al., 2003, Zhang et al., 2007a, 2009a, 2009b; Miao et al., 2008; Jian et al., 2010). In the

CABO, the closure of the Mongol–Okhotsk orogen shows a youngering trend from the

Permian–Jurassic in the west to Late Jurassic–Early Cretaceous in the east (Zonenshain et al., 1990; Cogné et al., 2005; Tomurtogoo et al., 2005).

During the Mesozoic, the Mongolian Plateau and surroundings have undergone strong intra-continental deformation (e.g., Zheng et al., 1991, 1996; Davis et al., 1998,

2001; Darby et al., 2001; Darby and Ritts, 2002; Wang et al., 2011 and references therein; Faure et al., 2012; Dong et al., 2015). The ongoing convergence of the Indian and Eurasian plates during the Cenozoic has greatly influenced much of the Asian continent from the southern Himalayan front to the northern SC (e.g., Molnar and

Tapponnier, 1975; Yin and Harrison, 2000).

The Mongolian Plateau is thus an ideal place to study intra-continental deformation in Asia during the Meso-Cenozoic. The formation of the Mongolian

Plateau may be related to the India–Eurasia collision (e.g., De Grave et al., 2007;

Vassallo et al., 2007) or from the interaction of a mantle plume with the continental lithosphere (e.g., Windley and Allen, 1993; Huang et al., 2015). Recent studies suggest that the plateau has been tectonically uplifted since the Early and Middle Jurassic and

its current altitude has been preserved since the last tectonic event at around 8 Ma

(Jolivet et al., 2007), but a large quantity of apatite fission track (AFT) data also indicate rapid uplift and erosion across the Mongolian Plateau during the Mesozoic (Fig.

1). The Mesozoic uplift of the Mongolian Plateau may be related to the

Mongol–Okhotsk orogeny, the sequential collision of the Qiangtang and Lhasa blocks with Eurasia, and the subduction of the Paleo-Pacific plate, or their combined impacts

(e.g., De Grave and Van den Haute, 2002; Li et al., 2011; Glorie et al., 2012).

The Langshan Mountains in the western Inner Mongolia, China, are the westernmost part of the Yinshan–Yanshan orogenic belt in the southern margin of the

Mongolian Plateau (Fig. 1; Darby and Ritts, 2007; Davis et al., 2010), and thus a joint thermochronlogical study on the Langshan Mountains will provide insights into the formation and evolution of the Mongolian Plateau.

2. Geological setting

Topographically, the Langshan Mountains, with an average elevation of

1500–2000 m in most parts, gently decrease northward and northwestward, in contrast with the significant southward decreases from c. 2000 m near Huogeqi to c. 1000 m near Qingshan and Urad Houqi in the Jilantai-Hetao basin (Figs. 1 and 2).

Except for widespread distibutions of Mesozoic and Cenozoic sedimetary sequences and local outcrops of Late Paleozoic sedimentary sequences (Fig. 2;

BGMRNMAR, 1991), the central part of the Langshan Mountains comprises

Neoarchean to Paleoproterozoic gneiss, Meso-Neoproterozoic metasedimentary rocks, and Late Paleozoic (Fig. 2; Wu et al., 2013; Wang et al., 2015, 2016). The youngest

pluton was emplaced at 236 ± 2 Ma (zircon U-Pb age, unpublished data).

In the southeastern piedmont of the Langshan Mountains, the Mesoproterozoic basement rocks are unconformably overlain by Jurassic or Cretaceous sequences (Fig.

2; BGMRNMAR, 1991). The Jurassic sequences are >1 km in thickness and show finer upward, whereas >2.5 km thick Cretaceous strata are composed of conglomerate, sandstone, and shale. The conglomerate clasts include non-foliated granitoids, gneiss, schist, and sandstone, indicating a proximal source (Darby and Ritts, 2007).

The pre-Mesozoic rocks are crosscut by major NE-striking thrusts, which are locally transected by subordinate NEN or NWN strike-slip faults (Fig. 2). Thrust F42 in the southeastern piedmont of the Langshan Mountains occurs in the Jurassic-Lower

Cretaceous sequences, but it was active during the Late Jurassic to Early Cretaceous

(Darby and Ritts, 2007). In the interior of the Langshan Mountains, thrust F13 was active before the Late Cretaceous (see Fig. 2). Along thrusts F32 and F33, a Triassic pluton (zircon U-Pb age of 241 ± 1 Ma; unpublished data) and its older country rocks were thrusted over the Lower Cretaceous sequences (Fig. 2). Both of the thrusts F35 and F38 crosscut the granitic plutons with a zircon U-Pb age of 236 ± 2 Ma

(unpublished data). Therefore, thrusts F42, F32, F33, F35, and F38 may have been active since the Triassic, except for the pre-Late Cretaceous thrust F13. In addition, a

Pliocene-Quaternary active mountain-front normal fault, as part of the Ordos graben system (Zhang et al., 1998), separates the Langshan Mountains from the Jilantai-Hetao

Basin (Fig. 2), in which more than 10 km thick fluvio-lacustrine clastic sediments have been deposited during the Cenozoic (SSBC, 1988).

3. Sampling and analytical procedures

3.1. Sampling

Thirty one samples were collected along two transects across the Langshan

Mountains and major thrusts, and other two samples were from the Bayin Qiandamen

(BQ) area in the north of the Langshan Mountains (Figs. 2 and 3; Table 1).

The HQ transect from Huogeqi to Qingshan traverses the most rugged part of the

Langshan Mountains (Fig. 2), and can be divided by thrusts F33, F35, and F38 into northern, middle, and southern parts (Figs. 3 and 5a). Eighteen granitic samples with zircon U-Pb ages from 297 ± 2 to 236 ± 2 Ma (unpublished data) were collected between 1227 and 2102 m (ASL) and performed for AFT analysis. Therein, two samples CG13-11 and CG13-15 were also used for biotite and K-feldspar 40Ar/39Ar dating and four samples 10LS-03, 10LS-17, CG13-13, and CG13-15 for apatite

(U-Th)/He (AHe) analysis (Fig. 3a; Table 1).

Along the CU transect from Chaoge Ondor to Urad Houqi (Fig. 2), seven granitic samples with zircon U-Pb ages from 269 ± 2 to 241 ± 1 Ma (unpublished data) and six

Precambrian granitic gneiss samples were collected between 1112 and 1839 m (ASL).

All samples were used for AFT analysis. Therein, one sample CG13-02 was selected for biotite and K-feldspar 40Ar/39Ar dating and another sample CG12-20 for muscovite and K-feldspar 40Ar/39Ar dating (Table 1). The current watershed is located between samples CG12-14 and CG12-28 and the CU transect is divided into northern and southern parts (Figs. 3a and 5b).

In addition, two granitic samples, with a zircon U-Pb age of 262 ± 3 Ma

(unpublished data), were collected at 1517 and 1519 m (ASL) in the BQ, northern part of the Langshan Mountains, where the topography is relatively flat (Fig. 2). Only AFT analysis was conducted on both samples (Table 1).

3.2. Analytical procedures

Biotite, muscovite, K-feldspar, and apatite grains were separated using the standard procedures, including crushing, sieving and heavy liquid and magnetic separation methods. Biotite/Muscovite and K-feldspar 40Ar/39Ar and AFT analyses were conducted in the Key Laboratory of Orogenic Belts and Crustal Evolution, at

Peking University. AHe analysis was prepared at Fission Track Lab in the University of

Glasgow and measured in the Scottish Universities Environmental Research Centre

(SUERC), respectively. Individual techniques are described in detail in the

Supplementary Text 1.

Each sample is modelled individually using QTQt software (Gallagher, 2012) in order to extract its thermal history. The modelling space is randomly sampled using the

Markov Chain Monte Carlo (MCMC) algorithm. The multi-compositional annealing model with c-axis projected length (Ketcham et al., 2007a, 2007b) is adopted for fission track annealing. The initial mean track length is all determined using the mean Dpar value of each sample with the formula suggested by Carlson et al. (1999). For the samples which also have AHe data, all single-grain raw AHe ages are jointly inverted using the radiation damage accumulation and annealing model of Gautheron et al.

(2009). If necessary, a set of samples from different elevations is modelled together

(joint modelling) to represent a quasi-vertical profile, because the joint modelling can

average out the complex deviations of individual samples, and the resultant thermal histories may simultaneously fit all samples from the vertical profile (Gallagher, 2012).

For individual modelling, the modelling space is simply constrained by the sample fission track age ± sample fission track age on time and 70 ± 70 °C on temperature. The present-day temperature is set to 10 ± 10 °C. Whenever possible, additionally independent geological constraints are added to improve modelling results. Individual modelling of samples CG13-11, CG13-15, and CG12-20 is constrained by their biotite/muscovite and K-feldspar 40Ar/39Ar data and the zircon U-Pb age of the youngest pluton (236 ± 2 Ma). The MCMC is run for at least 100,000 iterations for post-burn-in and burn-in, respectively.

For joint modelling of vertical profiles, an additional model parameter, i.e., present-day temperature offset between the top and bottom sample, should be taken into account. The present-day surface heat flow in the Langshan Mountains is estimated to be 40–60 mW/m (Lysak, 2009), similar to those in the surroundings of the Langshan Mountains (Hu et al., 2001), and thus the Langshan Mountains may also have the same present geothermal gradient of 20 to 30 °C/km as those in the nearby area (Hu et al., 2001). Therefore, a large range of possible geothermal gradient value of

25 ± 25 °C/km is used to calculate the present-day temperature offset. Joint modelling all starts from the latest magmatic activity (236 ± 2 Ma) and, if applicable, is constrained by biotite/muscovite and K-feldspar 40Ar/39Ar cooling ages. The MCMC is run at least 200,000 iterations for burn-in and 800,000 iterations for post-burn-in in order to better deal with a larger number of the input data and a higher complexity of the models.

The output of QTQt modelling is a collation of all thermal history models with

associated posterior probabilities. The expected model, which is adopted here, retains well constrained and robust features and averages out more complex T-t deviations observed in only a small number of possible models. More details about thermal history modelling approaches using QTQt can be found in Wildman et al. (2015).

4. Results

4.1. Biotite/muscovite and K-feldspar 40Ar/39Ar ages

Two samples from the HQ transect and one sample from the CU transect were performed for biotite and K-feldspar 40Ar/39Ar dating, and one sample from the CU transect was used for muscovite and K-feldspar 40Ar/39Ar dating (Table 1). The biotite/muscovite and K-feldspar 40Ar/39Ar data (Supplementary Text 2 and

Supplementary Table 1) all show a disturbed age spectrum (Fig. 4). Except for

K-feldspar separates from one sample (CG13-02), the other biotite/muscovite and

K-feldspar separates all yielded 40Ar/39Ar plateau ages (Fig. 4). Four biotite/muscovite

40Ar/39Ar plateau ages range from 205 ± 1 to 161 ± 1 Ma (Fig. 4), and three K-feldspar

40Ar/39Ar plateau ages vary from 167 ± 1 to 148 ± 1 Ma (Fig. 4). Seven 40Ar/39Ar plateau ages are generally consistent with their corresponding isochron ages.

4.2. AFT

AFT ages of 33 samples all passed the χ2 test (P (χ2) > 5%) and all the measured grains in each sample show similar annealing kinetic properties (Dpar values) (e.g.,

O’Sullivan and Parrish, 1995; Carlson et al., 1999; Ketcham et al., 2007b). AFT ages of single grains in each sample are thus classified as one population and expressed by the

AFT central ages (Green, 1981).

4.2.1. The HQ transect

In the HQ transect, eighteen AFT ages range from 184 ± 11 to 85 ± 7 Ma (Fig. 5a;

Table 2) and are significantly younger than the emplacement age of the youngest pluton

(236 ± 2 Ma). In the northern part to the northwest of thrust F33, four AFT ages of 184

± 11 to 164 ± 11 Ma display a youngering trend towards northwest and show no sharp age-jumps across thrust F13 (Fig. 5a; Table 2). In the middle part between thrusts F33 and F35, nine AFT ages vary from 158 ± 19 to 91 ± 7 Ma and decrease monotonously towards southeast until an age-jump across thrust F35 (Fig. 5a; Table 2). In the southern part between thrusts F35 and F38, the other five AFT ages of 114 ± 5 to 85 ± 7 Ma again decrease towards southeast until thrust F38 (Fig. 5a; Table 2).

Along this transect, seventeen samples have similar Dpar values of 1.4–1.9 μm, except for sample 10LS-3 with a longer Dpar value of 3.2 μm (Table 2). The mean track lengths (MTLs) are in ranges of 11.6 ± 1.9 to 13.1 ± 1.4 μm in three samples of the northern part, 11.1 ± 1.8 to 12.7 ± 1.8 μm in nine samples of the middle part, and 11.8 ±

1.6 to 12.4 ± 1.4 μm in three samples of the southern part (Table 2). Track length distributions of each sample display unimodal and negatively skewed patterns (Figs.

7–9, SF1, and SF2).

4.2.2. The CU transect

Thirteen samples from the CU transect yielded AFT ages of 147 ± 11 to 79 ± 4 Ma

(Fig. 5b; Table 2). Similarly, AFT ages show a youngering trend from 147 ± 11 to 92 ±

8 Ma in the northern part and from 118 ± 7 to 79 ± 4 Ma in the southern part until thrust 42 as elevations decrease, but the four samples from a quasi-vertical profile show increasing AFT ages of 108 ± 5 to 116 ± 5 Ma with lowering elevations (Fig. 5b;

Table 2).

All the samples, however, have similar Dpar values of 1.2–1.6 μm (Table 2), and their track length distributions are negatively skewed and unimodal, with the MTLs varying from 12.4 ± 1.4 to 12.7 ± 1.3 μm (Figs. 10, 11, SF3, and SF4; Table 2).

4.2.3. The BQ

Twenty one and 26 countable apatite grains in samples CG12-01 and CG12-02 from the BQ yielded AFT ages of 147 ± 8 and 136 ± 7 Ma, respectively (Fig. 5c). The two samples have identical Dpar values of 1.2 μm and their track length distributions are unimodal, with the MTLs of 12.4 ± 1.4 and 11.9 ± 1.5 μm, respectively (Fig. 13;

Table 2).

4.3. AHe

AHe analysis was performed only on four samples from the HQ transect (Table

3), and their raw single-grain AHe ages are in ranges of 134 ± 8 to 87 ± 5 Ma for

sample 10LS-03, 112 ± 7 to 29 ± 2 Ma for sample CG13-13, 113 ± 7 to 63 ± 4 Ma for sample CG13-15, and 60 ± 4 to 21 ± 1 Ma for sample 10LS-17.

Raw single-grain AHe ages from one single sample have large degrees of dispersion, ranging from 43 to 130% (Table 3). Samples 10LS-3 and CG13-15 show a positive correlation between raw single-grain AHe ages and crystal width (Figs. 6a and

6e), but samples CG13-13 and 10LS-17 show no clear correlations (Figs. 6c and 6g).

Furthermore, the AHe ages and (eU) content in all four samples show no linear relationship (Figs. 6b–6h).

4.4. Summary of joint thermochronological data

It is clear that the ages obtained from different minerals are consistent well with their closure temperatures (Table 4), suggesting the sequential cooling of the dated minerals. Most of the AFT samples show a positive correlation between ages and elevations (Fig. 5), but the four samples from the quasi-vertical profile in the southern part of the CU transect display an inverted age-elevation trend (Fig. 5b). Such an abnormal age-elevation relationship is probably due to rapid stream trenching other than faulting, just like that of the glacial erosion in the Namche Barwa Peak, eastern

Himalayan syntaxis (Tu et al., 2015). However, these four AFT ages are compatible with others along the southern part of the CU transect. Therefore, all of the 40Ar/39Ar,

AFT, and AHe data can be used for thermal history modelling.

5. Thermal history modelling results

Out of 33 AFT samples, 25 samples have sufficient confined fission tracks

(46–188) for individual modelling, but all the 33 samples were treated as three vertical profiles in the northern, middle, and southern parts of the HQ transect and two profiles in the northern and southern parts of the CU transect in order to perform joint modelling. Results from joint modelling and representative individual modelling are displayed in Figs. 7–12, and the other individual modelling results can be found in Figs.

SF1–SF4.

5.1. The HQ transect

After the emplacement of the Middle Triassic granitic pluton, the thermal history of the northern part of the HQ transect may be divided into three stages (Fig. 7): (1) rapid cooling from c. 115 to c. 25 °C during c. 220 and c. 100 Ma (c. 0.75 °C/Ma); (2) little cooling between c. 100 and c. 15 Ma, and (3) relatively rapid cooling from c.

40 °C to the current surface temperature after c. 15 Ma (c. 2.0 °C/Ma).

The thermal history of the middle part of the HQ transect, after cooling of the pluton to the closure temperature of c. 250 °C for K-feldspar 40Ar/39Ar age, may be divided into three stages (Fig. 8): (1) rapid cooling from 250 to 75–50 °C between c.

170 and 130 Ma (c. 4–5 °C/Ma and subsequently slow cooling from 85–60 to 40–20 °C during 120 and 80 Ma (c. 1.0 °C/Ma), (2) little cooling until c. 10 Ma, and (3) relatively rapid cooling from 65–40 °C to the current surface temperature after c. 10 Ma (c.

3.0–5.5 °C/Ma).

Similarly, the southern part of the HQ transect is also characterized by three cooling stages (Fig. 9): (1) rapid cooling from c. 350 to c. 70–80 °C during 170 and 130

Ma (c. 7.0 °C/Ma) and subsequently slow cooling from 90–80 to 35–25 °C between

120 and 80 Ma (1.4 °C/Ma), (2) little cooling until c. 10 Ma, and (3) relatively rapid cooling from 55–45 °C to the current surface temperature after c. 10 Ma (c.

3.5–4.5 °C/Ma).

5.2. The CU transect

In the northern part of the CU transect, the thermal history was also divided into three stages after the emplacement of the Middle Triassic granitic pluton (Fig. 10): (1) slow cooling from c. 120 to c. 60 °C during 160–100 Ma (c. 1.0 °C/Ma); (2) little cooling until c. 12 Ma; and (3) relatively rapid cooling from 50–40 °C to the current surface temperature since c. 12 Ma (c. 2.5–3.3 °C/Ma).

The southern part of this transect also revealed a three-stage thermal history after the cooling of the pluton to the closure temperature of c. 325 °C for muscovite

40Ar/39Ar age (Fig. 11): (1) rapid cooling from c. 325 °C to c. 50 °C between 160 and

100 Ma (c. 4.6 °C/Ma); (2) little cooling until c. 15 Ma; and (3) relatively rapid cooling from 55–45 °C to the current surface temperature since c. 15 Ma (c. 2.3–3.0 °C/Ma).

5.3. The BQ

The BQ is also featured by three stages of cooling following the emplacement of the Middle Triassic granitic pluton (Fig. 12): (1) slow cooling from c. 100 to c. 40 °C between c. 200 and c. 100 Ma (c. 0.6 °C/Ma); (2) little cooling between c. 100 and c. 20

Ma, and (3) relatively rapid cooling from c. 40 °C to the current surface temperature

after c. 20 Ma (c. 1.5 °C/Ma).

5.4. Assessment of the modelling qualities

Predicted AFT data of all samples from thermal history modelling are within 80% of the observed values, except for two samples (CG12-18 and CG12-20; Figs. 7c, 8c, 9c,

10c, 11c, and 12; see Figs. SF1–SF44 for more individual modelling predictions).

However, the predicted AHe ages of about half dated grains are within 80% of the measured ages (Figs. 7c, 8c, 9c, 10c, and 11c; see Figs. SF1–SF4 for more individual modelling predictions). Wildman et al. (2016) explained the poor ﬁt of the AHe data relative to that of AFT data.

6. Discussion

The joint thermal history modelling revealed that the Langshan Mountains has experienced three-stage cooling since the Late Triassic, which may be outlined as follows: (1) contrasting cooling in different parts before c. 100–80 Ma, (2) little cooling between c. 100–80 and c. 20–10 Ma, and (3) rapid cooling from 65–40 °C to the present-day surface temperature since c. 15 Ma (c. 1.5–5.5 °C/Ma). In particular, the last two stages of cooling have been recorded throughout the whole Langshan

Mountains.

6.1. Exhumation of the Langshan Mountains

For the first stage of cooling, the BQ and the northern parts of the HQ and CU transects show similar cooling at c. 0.6–1.0 °C/Ma, in contrast to that in the middle and southern parts of the HQ transect and in the southern part of the CU transect. The first stage in the southern part of the CU transect is characterized by rapid cooling (at c.

4.6 °C/Ma) during 160–100 Ma only (Fig. 11), whareas two sub-stages are identified in the middle and southern parts of the HQ transect: early rapid cooling (c.

4.0–7.0 °C/Ma) during 170–130 Ma and following slow cooling (c. 1.0–1.4 °C/Ma) during 120–80 Ma (Figs. 8 and 9).

It is estimated that the first-stage cooling started at c. 220 Ma, which was postponed to the youngest plutonism at 236 Ma, so that the cooling revealed by joint thermal history modelling has resulted mainly from denudation. The amount of denudation in each stage can be calculated assuming two geothermal gradients of 30 and 20 °C/km for rapid and slow denudation, respectively, because rapid denudation will enhance the geothermal gradient (e.g., Braun, 2002). Consequently, the BQ and the northern parts of the HQ and CU transects underwent denudation of c. 2–3 km

(160–100 Ma), significantly lower than denudation of c. 6–9 (170–130 Ma) plus c. 2–3

(120–80 Ma) km in the middle and southern parts of the HQ transect and c. 9 km

(160–100 Ma) in the southern part of the CU transect (Fig. 3a). If the current average geothermal gradient is 25 °C/km, an amount of denudation c. 1.2–2.2 km is estimated for the last stage since c. 15 Ma.

The denudation of c. 6–9 km in the interior part of the Langshan Mountains during 170–100 Ma is partially recorded by the >3.5 km thick Jurassic to Early

Cretaceous clastic sequences in the southeastern piedmont of the Langshan Mountains

(Fig. 2), in which the conglomerate clasts include Precambrian basement rocks and

granitic rocks, suggesting a local source for the sediments (Darby and Ritts, 2007).

6.2. Impacts of thrusting on denudation of the Langshan Mountains

It is known that five major thrusts develop in the Langshan Mountains, especially in the area transversed by the HQ transect (Figs. 2 and 3), and those thrusts would have affected the Meso-Cenozoic denudation of the Langshan Mountains. Although cooling of rocks does not directly arise from thrusting (e.g., Platt, 1993), the variations of AFT data in a cross-section may be related to thrust faulting in the case of sufficiently rapid erosion (e.g., Lock and Willett, 2008). Thrusting first uplifts rocks on the hanging wall. The materials above the erosional surface would be removed and underlying rocks would cool down, which may be recorded by different thermochronological systems. The thrusting may result in the tilting of thrust sheets and exposure of deeper rocks so that the magnitude of uplift and denudation become larger towards thrusts (e.g., ter Voorde et al., 2004; Lock and Willett, 2008; Metcalf et al., 2009), accompanied by a younger trend of AFT ages (see Figs. 5 and 13). Similar cases occur also in the Bogeda Mountain of the Chinese Tian Shan (Tang et al., 2015) and the Longmenshan in the eastern margin of the Tibetan Plateau (Tan, 2012). In the present case, the youngering trend of AFT ages towards thrusts and large denudation of c. 6–9 km in the middle and southern parts of the HQ transect and the southern part of the CU transect might have been related to thrusting.

Detailedly, the four AFT ages from both sides of thrust F13 show a consistently decreasing trend with elevations, without a sharp age-jump across thrust F13 (Figs. 5a and 14a), implying that the thrusting was little when it was active before the Late

Cretaceous (Fig. 2), and thus the hanging and foot walls show slow cooling (c.

0.6–1.0 °C/Ma) during 220–100 Ma, with similar denudation of c. 2 km on both sides of thrust F13 from 160 to 100 Ma (Figs. 3a and 7). Similarly, there are no age-jumps across thrust F33 (Figs. 5a and 14a), suggesting that thrust F33 had little contribution to denudation of the middle part from 170 to 130 Ma (Figs. 3a and 8). However, the middle and southern parts show a monotonous decrease in AFT ages southeastward until the northwest-dipping thrusts F35 and F38 (Figs. 5a and 14a), with an age-jump across thrust F35 and larger denudation of c. 9 km in the southern part than that (c. 6–7 km) in the middle part during 170–130 Ma (Figs. 3a and 9). Generally, the decreasing

AFT ages and increasing denudation from north to south should be controlled by F35 and F38 thrusting and the resultant tilting of the thrust sheets (Fig. 3b).

In addition to the similar patterns of AFT ages with elevations along the HQ and

CU transects, both transcets also show comparable denudation in the first stage (see

Fig. 3 for details), but the patterns of AFT ages with elevations and denudation in the southern part of the CU transect may have been controlled only by thrust F42.

Apparently, thrusts F35, F38, and F42 have had important impacts on the denudation of the Langshan Mountains during 170–100 Ma. This is in agreement with the contractional period in the whole Yinshan–Yanshan orogenic belt during the Middle

Jurassic to Early Cretaceous (e.g., Zhang et al., 2007b; Lin et al., 2013).

6.3. Implications for the Mongolian Plateau

In the Langshan Mountains, the rapid cooling and denudation occurred mainly before 100–80 Ma. The AFT data available from the literature clearly show that the

Late Triassic to Cretaceous AFT ages have been reported from most parts of the

Mongolian Plateau (Fig. 1 and references therein) and adjacent areas, such as the

Kyrgyz Northern Tien Shan (Degrave et al., 2013), the central Chinese Tian Shan

(Jolivet et al., 2010), and the northern part of the Tibetan Plateau (Jolivet et al., 2001).

It is very interesting to note that the Cretaceous and younger AFT ages are principally confined to the peripherals of the plateau, such as the Baikal area, the Sayan

Mountains, the Altay Mountains, the Beishan Mountains, the Langshan Mountains, the

Daqingshan Mountains, the Yanshan Mountains, and the Greater Hinggan Mountains, with a youngering trend outward (Fig. 1). This implies that the outward growth of the

Mongolian Plateau has occurred since the Mesozoic.

Temporally, the Mesozoic uplift and denudation of the Mongolian Plateau might be generally simultaneous with the docking of the Qiangtang and Lhasa blocks to the southern margin of Eurasian continent during the Late Triassic to Early Cretaceous

(e.g., Pullen et al., 2008; Yin and Harrison, 2000), and the scissor-like closure of the

Mongol–Okhotsk Ocean from west to east from Permian–Jurassic to the Early

Cretaceous (e.g., Zonenshain et al., 1990; Cogné et al., 2005; Tomurtogoo et al., 2005), and the subduction of the Paleo-Pacific plate during the Early Jurassic–Cretaceous (e.g.,

Maruyama, 1997; Zhou et al., 2006). However, the latest uplift of the Langshan

Mountains since c. 15 Ma is coeval with the exhumation of the Elashan Mountains at c.

15 Ma (Duvall et al., 2013) in the northern Tibetan Plateau and the Liupanshan

Mountains at c. 8 Ma (Zheng et al., 2006) and Helanshan Mountains at c. 12 Ma (Liu et al., 2010) around the northeastern Tibetan Plateau, probably due to the distant effect of rapid uplift in the northern part of the Tibetan Plateau at c. 15 Ma (Lu et al., 2016).

7. Conclusions

Multiple thermochronological methods were applied to the samples from the HQ and CU transects and the BQ of the Langshan Mountains. Four biotite/muscovite

40Ar/39Ar plateau ages range from 205 ± 1 to 161 ± 1 Ma, three K-feldspar 40Ar/39Ar plateau ages vary between 167 ± 1 and 131 ± 1 Ma, 33 AFT ages range from 184 ± 4 to

79 ± 11 Ma, and 31 single-grain raw AHe ages range between 134 ± 8 to 21 ± 1 Ma.

Thermal history modelling using joint thermochrnological data indicates a three-stage cooling history in the Langshan Mountains: (1) rapid cooling with spatial differentiation in different parts at distinct rates before 100–80 Ma, (2) little cooling in the whole Langshan Mountains between 100–80 and 20–10 Ma, and (3) rapid cooling

(c. 1.5–5.5 °C/Ma) to the present-day surface temperature also in the whole Langshan

Mountains since c. 15 Ma. In the first stage, cooling at c. 0.6–1.0 °C/Ma was recorded in the northern parts of the HQ and CU transects and the BQ from 220–200 or 160 to

100 Ma, where an amount of denudation was c. 2–3 km during 160–100 Ma. In the middle-southern parts of the HQ transect and the southern part of the CU transect, the

AFT ages become younger towards thrusts F35, F38, and F42, and rapid cooling (c.

4.0–7.0 °C/Ma) and remarkable denudation of c. 6–9 km from 170–130 or 160–100 Ma were probably controlled by the thrusting and the thrust-sheet tilting of F35, F38 and

F42, respectively. The final stage, with the denudation of <2 km, is coeval with the rapid uplift of the northern part of the Tibetan Plateau at c. 15 Ma. Moreover, the AFT ages throughout the Mongolian Plateau show a youngering trend from the inner to the peripherals, suggesting the outward propagation of the Mongolian Plateau since the

Mesozoic.

Acknowledgements

We would like to thank Mark Wildman (University of Glasgow) for double check of apatite grains and Zhongpeng Han (Edingburgh University, a visitor from China

University of Geosciences Beijing) for (U-Th)/He analysis. We appreciate Professor

Fin Stuart (Scottish Universities Environmental Research Centre) for his advice for

AHe data and thermal history modelling. We are also grateful to two anonymous reviewers for their comments and suggestions. This work was financially supported by the Ministry of Science and Technology of China (Grant No. 2013CB429806).

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Figure captions

Fig. 1. Simplified tectonic map of the Mongolian Plateau (after Zhang et al. 1998;

Vassallo et al., 2007; Wang et al., 2011; Li et al., 2016; AFT ages from Van de Beek et al., 1996; Wu et al., 2000; De Grave and Van den Haute, 2002; Wu and Wu, 2003; Yuan et al., 2006; De Grave et al., 2007, 2011, 2014; Jolivet et al., 2007; Vassallo et al., 2007;

Liu et al., 2010; Li et al., 2011, 2016; Glorie et al., 2012; Gillespie et al., 2015).

Fig. 2. Simplified geological map of the Langshan Mountains (after

BGMRNMAR, 1991; Zhang et al. 1998). Numbers in boxes show zircon U-Pb ages.

Fig. 3. (a) Samples with the thermochronological ages along the HQ and CU transects (after BGMRNMAR, 1991). Rapidly denudated parts are shown in gray. (b)

Schematic cross-section of the HQ transect (after Darby and Ritts, 2007) showing the impacts of thrusts on denudation of the Langshan Mountains in the first stage.

Fig. 4. Biotite/muscovite and K-feldspar 40Ar/39Ar plateau ages of four samples from the HQ and CU transects. Solid boxes were used for the plateau ages calculation.

Fig. 5. Simplified geological cross-sections of the HQ and CU transects and the

BQ with thermochronological ages. See Figs. 2 and 3a for symbols.

Fig. 6. Relationship between single-grain raw AHe ages and grain width as well as the effective uranium (eU).

Fig. 7. Thermal history modelling in the northern part of the HQ transect. Black vertical lines show the cooling events interpreted. (a) T-t paths from joint modelling. (b)

Zoom of the red box in figure a. (c) Relationship of AFT parameters and AHe ages versus elevations. (d) T-t paths from representative individual modelling. The color map indicates the probability distribution of tested models at a resolution of one Myr. (e)

Observed track length distribution together with the measurements and predictions of the AFT and AHe parameters.

Fig. 8. Thermal history modelling in the middle part of the HQ transect. (a) T-t paths from joint modelling. (b) Zoom of the red box in figure a. (c) Relationship of AFT parameters and AHe ages versus elevations. (d) T-t paths from representative individual modelling. (e) Zoom of the red box in figure d. (f) Observed track length distribution together with the measurements and predictions of AFT and AHe parameters. See Fig.

7 for symbols.

Fig. 9. Thermal history modelling in the southern part of the HQ transect. (a) T-t paths from joint modelling. (b) Zoom of the red box in figure a. (c) Relationship of AFT parameters and AHe ages versus elevations. (d) T-t paths from representative individual modelling. (e) Zoom of the red box in figure d. (f) Observed track length distribution together with the measurements and predictions of AFT and AHe parameters. See Fig.

7 for symbols.

Fig. 10. Thermal history modelling in the northern part of the CU transect. (a) T-t

paths from joint modelling. (b) Zoom of the red box in figure a. (c) Relationship of AFT parameters versus elevations. (d) T-t paths from representative individual modelling. (e)

Observed track length distribution together with the measurements and predictions of

AFT parameters. See Fig. 7 for symbols.

Fig. 11. Thermal history modelling in the southern part of the CU transect. (a) T-t paths from joint modelling. (b) Zoom of the red box in figure a. (c) Relationship of AFT parameters versus elevations. (d) T-t paths from representative individual modelling. (e)

Zoom of the red box in figure d. (f) Observed track length distribution together with the measurements and predictions of AFT parameters. See Fig. 7 for symbols.

Fig. 12. Individual thermal history modelling in the BQ. See Fig. 7 for symbols.

Fig. 13. Thrusting, tilting and exhumation pathways through the biotite/muscovite,

K-feldspar, and AHe closure temperatures and AFT PAZ. Particularly, AFT ages of samples A, B, and C, which come from the un-annealed, partially annealed, and fully annealed zones, respectively, show a youngering trend towards the thrust. Black fine lines are references and black thick line indicates listric thrust fault. Tc: closure temperature.

Fig. 14. AFT age-elevation relationship in the HQ (a) and CU transects (b).

Table 1. Sample details and methods used in this study.

Table 2. Apatite fission track data in this study.

Table 3. Results of apatite (U-Th)/He analysis.

Table 4. Summary of the joint thermochronological data in this study.

Fig. SF1. Individual thermal history modelling in the HQ transect. See Fig. 7 for symbols.

Fig. SF2. Individual thermal history modelling in the HQ transect. See Fig. 7 for symbols.

Fig. SF3. Individual thermal history modelling in the CU transect. See Fig. 7 for symbols.

Fig. SF4. Individual thermal history modelling in the CU transect. See Fig. 7 for symbols.

Supplementary Table 1. Full summary of conventional 40Ar/39Ar data

(uncertainties are reported at 2σ).

Supplementary Text 1. Multiple thermochronological analytical procedures.

Supplementary Text 2. Biotite/muscovite and K-feldspar 40Ar/39Ar ages.

Table 1

Sample details and methods used in this study.

S R Plut Ele Lat Lo Loca Met ample ock on age vation itude ngitude lity hod no. type (Ma) (m)

Gr N4 E1 The 1 270 19 AFT anodior 1° 06°38′3 HQ 0LS-3 ± 1 55 , AHe ite 12′52.7″ 1.0″ transect

N4 E1 The 1 Gr 259 20 1°08′39. 06°36′4 HQ AFT 0LS-5 anite ± 5 34 2″ 4.1″ transect

C N4 E1 The Gr 259 21 G13-0 1°08′17. 06°36′4 HQ AFT anite ± 5 02 9 9″ 8.1″ transect

N4 E1 The 1 Gr 259 20 1°08′08. 06°36′4 HQ AFT 0LS-6 anite ± 5 52 0″ 2.4″ transect

C N4 E1 The Gr 236 19 G13-1 1°06′32. 06°36′2 HQ AFT anite ± 2 02 0 1″ 5.1″ transect

C N4 E1 The Gr 236 18 G12-2 1°06′46. 06°36′2 HQ AFT anite ± 2 65 6 7″ 5.1″ transect

C Gr 236 N4 E1 18 The B,

G13-1 anite ± 2 1°06′26. 06°37′1 42 HQ K, AFT

1 5″ 9.9″ transect

N4 E1 The 1 Gr 236 17 1°06′15. 06°38′1 HQ AFT 0LS-9 anite ± 2 38 7″ 9.8″ transect

C N4 E1 The Gr 236 17 G13-1 1°06′10. 06°38′2 HQ AFT anite ± 2 85 2 9″ 7.6″ transect

1 N4 E1 The Gr 236 16 0LS-1 1°05′32. 06°38′3 HQ AFT anite ± 2 30 0 7″ 5.7″ transect

C N4 E1 The Gr 236 16 AFT G13-1 1°04′38 06°38′5 HQ anite ± 2 77 , AHe 3 ″ 0.6″ transect

C N4 E1 The Gr 236 15 G13-1 1°03′54. 06°42′6. HQ AFT anite ± 2 70 4 5″ 8″ transect

1 N4 E1 The Gr 240 15 0LS-1 1°03′51. 06°42′0 HQ AFT anite ± 2 09 3 6″ 9.8″ transect

1 N4 E1 The Gr 240 14 0LS-1 1°03′31. 06°43′4 HQ AFT anite ± 2 54 4 6″ 1.7″ transect

1 Gr 240 N4 E1 14 The AFT

0LS-1 anite ± 2 1°02′54. 06°44′3 14 HQ

5 0″ 6.0″ transect

C N4 E1 The B, Gr 240 13 G13-1 1°02′27. 06°45′1 HQ K, AFT, anite ± 2 67 5 8″ 1.1″ transect AHe

1 N4 E1 The Gr 240 13 0LS-1 1°01′48. 06°45′2 HQ AFT anite ± 2 07 6 3″ 9.3″ transect

1 N4 E1 The Di 297 12 AFT 0LS-1 0°59′56. 06°46′5 HQ orite ± 2 27 , AHe 7 3″ 6.9″ transect

C N4 E1 The Gr 269 18 G12-0 1°21′16 07°3′50. CU AFT anite ± 2 39 9 ″ 2″ transect

C N4 The Gr 269 E1 16 G12-1 1°21′31. CU AFT anite ± 2 07°3′45″ 36 1 8″ transect

C N4 E1 The Gr 269 15 G13-0 1°21′15. 07°5′0.5 CU AFT anite ± 2 13 4 9″ ″ transect

C N4 E1 The Gr 269 14 G12-1 1°20′17 07°5′13. CU AFT anite ± 2 90 4 ″ 7″ transect

C G Prec N4 E1 16 The AFT

G12-2 neiss ambrian 1°15′19. 07°3′14. 40 CU

1 7″ 2″ transect

C N4 E1 The G Prec 15 G12-2 1°15′20. 07°3′21. CU AFT neiss ambrian 44 2 6″ 8″ transect

C N4 E1 The Gr 241 15 G12-2 1°18′30. 07°4′38. CU AFT anite ± 1 20 8 3″ 9″ transect

C N4 E1 The Gr 241 14 B, G13-0 1°18′5.9 07°4′33. CU anite ± 1 93 K, AFT 2 ″ 8″ transect

C N4 E1 The G Prec 14 G12-2 1°15′22. 07°3′31. CU AFT neiss ambrian 60 3 4″ 7″ transect

C N4 The G Prec E1 13 G12-1 1°15′27. CU AFT neiss ambrian 07°3′49″ 87 6 7″ transect

C N4 E1 The G Prec 13 G12-1 1°14′25. 07°5′5.7 CU AFT neiss ambrian 25 7 5″ ″ transect

C N4 E1 The G Prec 12 G12-1 1°13′33. 07°6′10. CU AFT neiss ambrian 53 8 3″ 3″ transect

C Gr 260 N4 E1 11 The M,

G12-2 anite ± 1 1°10′33. 07°8′34. 12 CU K, AFT

0 3″ 7″ transect

C N4 E1 Gr 262 15 The G12-0 1°46′31. 07°20′3 AFT anite ± 3 17 BQ 1 7″ 8.9″

C N4 E1 Gr 262 15 The G12-0 1°46′39. 07°21′4 AFT anite ± 3 19 BQ 2 5″ 4.1″

Except for the pluton age of sample CG12-20, which is from Wu et al. (2013), the other pluton ages are our unpublished data. B = Biotite 40Ar/39Ar analysis, M =

Muscovite 40Ar/39Ar analysis, K = K-feldspar 40Ar/39Ar analysis, AFT = Apatite fission track analysis, AHe = Apatite (U-Th)/He analysis.

Table 2

Fission track data in this study.

ρ ρ ρ T P M D Sa s iN dN N n TL parN 5 5 5 2 mple no. (10 /c s (10 /c i (10 /c d (Ma (χ (μm) (μm) m2) m2) m2) ) )

The

HQ transect

Nor thern part

6 1 1 1 10 7 5 7 7 1 1 8 3 49 64 ± .0 2.8 ± LS-3 .3 81 .3 36 0.4 4 4 .2 4 11 0 1.8

6 1 0 10 3 4 4 5 1 2 1 49 78 ± .9 LS-5 .6 69 .2 49 0.4 4 .4 4 13 9

8 1 7 1 0 1 CG 1 1 1 2 8 1 03 05 02 72 ± .9 1.6 ± 13-09 4.0 8.4 1.2 0 5 .9 9 2 5 10 1 1.9

6 1 1 1 10 1 7 1 8 1 2 6 1 49 84 ± .0 3.1 ± LS-6 0.2 75 1.5 76 0.3 0 8 .3 4 11 0 1.4

Mi ddle part

6 1 0 CG 5 1 7 1 1 1 1 64 58 ± .8 13-10 .1 34 .4 93 1.3 0 .6 5 19 2

6 1 0 1 CG 6 3 9 6 1 1 6 1 64 50 ± .9 2.4 ± 12-26 .3 87 .9 05 1.7 8 3 .3 5 11 9 1.5

1 7 1 0 1 1 CG 8 9 1 1 2 1 46 02 48 ± .9 2.3 ± 2 13-11 .3 63 2.6 1.2 4 .7 6 5 8 9 1.3 1

1 4 1 0 1 1 10 4 8 5 8 2 1 12 73 27 ± .9 2.7 ± 2 LS-9 .6 66 .9 .2 5 .5 7 6 7 2 1.8 7

7 1 0 1 CG 8 3 1 4 1 1 4 1 02 43 ± .8 2.3 ± 13-12 .8 12 3.8 92 1.2 7 6 .9 5 11 8 1.4

1 4 1 0 1 10 5 9 8 8 2 9 1 40 73 06 ± .9 1.1 ± LS-10 .5 10 .6 .1 8 7 .5 8 6 6 9 1.8

1 7 1 0 1 CG 3 6 7 1 2 8 1 22 02 21 ± .9 2.2 ± 13-13 .9 51 .3 1.3 0 4 .7 5 5 7 8 1.8

CG 8 2 1 4 1 7 1 2 0 1 1 1

13-14 .0 10 7.9 71 1.3 02 02 ± 0 .9 1.7 ± 2 .8

6 1 5 4 0 1.6 5

4 9 0 10 3 5 5 8 8 1 1 73 1 ± .9 LS-13 .3 60 .1 56 .0 1 .3 6 7 9

Sou thern part

1 4 1 1 10 5 5 9 7 1 1 02 73 00 ± .0 LS-14 .7 95 .7 .9 2 .4 0 6 7 0

2 3 4 1 0 10 1 1 7 2 1 33 07 73 14 ± .9 LS-15 0.6 3.9 .8 3 .5 4 9 6 5 6

1 3 7 1 0 1 1 CG 6 1 1 2 1 65 37 02 12 ± .9 2.2 ± 0 13-15 .4 3.0 1.3 0 .8 9 1 5 5 9 1.5 9

3 4 4 1 1 1 1 10 1 1 7 2 1 03 46 73 05 ± .0 1.8 ± 8 LS-16 2.1 7.8 .7 2 .6 1 4 6 4 0 1.6 8

4 8 1 1 10 5 3 1 6 7 2 7 1 73 5 ± .0 2.4 ± LS-17 .7 44 0.6 33 .6 1 2 .7 6 7 0 1.4

The

CU transect

Northern part

7 1 0 1 GG 3 3 5 5 1 2 9 1 80 47 ± .9 2.6 ± 12-09 .3 49 .5 84 2.2 2 6 .6 7 11 9 1.7

4 1 1 1 CG 3 4 4 6 7 3 7 1 63 04 ± .0 2.7 ± 12-11 .4 59 .7 48 .3 2 0 .4 8 7 0 1.3

7 9 0 CG 3 1 8 4 1 2 1 02 2 ± .8 13-04 .5 92 .7 73 1.2 1 .2 5 8 9

4 1 1 CG 2 4 4 6 7 9 1 4 1 63 .0 2.4 ± 12-14 .9 35 .3 47 .7 6 ±8 7 9 .3 8 0 1.2

Sou thern part

1 1 4 1 1 1 CG 4 5 7 3 6 1 34 88 79 08 ± .0 2.6 ± 12-21 .1 .7 .4 5 3 .2 0 3 8 5 0 1.3

CG 1 1 1 2 7 4 1 2 1 1 1 1

12-22 0.3 99 3.6 63 .5 79 14 ± 3 .0 2.4 ± 4 .3

2 6 8 5 0 1.2 2

4 1 1 CG 2 2 4 4 7 1 1 63 14 ± .0 12-28 .8 83 .2 28 .9 0 .4 8 11 0

1 7 1 1 CG 5 8 9 1 3 1 51 02 18 ± .0 13-02 .1 20 .4 1.1 3 .4 2 5 7 0

1 2 4 1 0 1 1 CG 1 1 7 2 1 66 22 63 14 ± .9 2.2 ± 3 12-23 0.5 4 .6 2 .2 3 0 8 5 5 1.2 0

2 3 4 1 1 1 1 CG 1 2 7 2 1 31 09 63 17± .0 2.3 ± 4 12-16 8.2 4.3 .7 8 .3 6 9 8 5 0 1.3 5

2 5 7 1 1 1 1 CG 1 3 1 2 1 47 30 80 04 ± .0 2.1 ± 6 12-17 7.9 8.5 1.8 0 .5 0 9 7 5 0 1.3 5

1 2 4 9 1 1 1 CG 1 1 7 3 1 17 03 63 1 ± .0 2.7 ± 2 12-18 1.0 9.0 .8 6 .3 4 0 8 5 0 1.3 5

1 4 7 1 1 CG 1 9 2 7 2 9 1 77 63 9 ± .0 2.6 ± 12-20 1.5 47 1.5 .3 2 3 .2 9 8 4 0 1.3

The BQ

CG 1 7 1 1 1 7 1 2 0 1 1 1

12-01 0.4 79 8.4 37 2.9 80 47 ± 1 .9 2.4 ± 3 .2

7 7 8 4 1.4 5

1 7 1 0 1 CG 1 8 2 1 2 9 1 67 80 36 ± .9 1.9 ± 12-02 1.1 89 0.9 2.8 6 2 .2 7 7 7 5 1.5

Note: ρs and ρi is spontaneous and induced fission track density, respectively; Ns and Ni is the number of spontaneous and induced fission tracks, respectively; ρd is interpolation of induced glass dosimeter CN5 track densities; Nd is the number of induced fission tracks in glass dosimeter CN5; T (± 1σ) is the AFT central age; n is the number of measured apatite grains in each sample; P (χ2) is the probability of equaling;

MTL (± 1σ) is the mean track length; N is the number of confined fission tracks in each sample; SD is the length standard deviation; Dpar is the mean etch pit figure diameter.

Table 3

Results of apatite (U-Th)/He analysis.

2 R Di 4 2 2 ( L W U G 35 aw spersio He 38U 32Th eU)a Tc c Cd rain U Age ne b S # ( (% (ppm) (Ma) ample cc) (μm) )

4 2 2 2 3 2 1 6 1 1 .9E-0 80.8 .0 06.3 31.4 7 T 62 5 11 9 8 4 5 1

2 1 0 1 9 2 1 7 1 2 .7E-0 10.7 .8 33.4 6 3.06 T 73 8 04 9 9 0 6

2 0 4 5 5 1 1 1 1 5 .9E-0 .3 7 1 6.03 4.13 9.08 T 68 07 24 9 3 42 0LS-3 1 1 0 1 7 2 1 6 8 6 .7E-0 08.0 .7 27.5 5 9.59 T 92 3 7 9 6 8 5

6 0 3 2 3 1 2 1 1 7 .2E-0 .2 7 0.14 2.19 5.57 T 27 06 21 9 2

5 1 0 1 1 1 1 9 1 8 6 .6E-0 31.8 .9 18.9 60.7 T 77 5 01

9 0 6 6 1

2 0 5 5 6 1 1 9 1 9 .7E-0 .4 6 5.44 0.72 7.76 T 90 5 06 9 0

1 1 0 1 1 9 1 1 1 1 .1E-0 16.1 .8 39.3 8 0 5.02 T 86 22 34 8 8 4 5

6 1 0 1 1 9 2 1 1 1 .2E-0 04.8 .7 27.3 7 1 2.31 T 95 10 21 9 6 6 1

1 0 2 1 2 2 2 1 1 2 .9E-0 .1 7 1.82 6.78 5.92 T 07 42 12 9 6

C 3 0 1 1 3 2 1 8 4 13 G13-1 4 .4E-1 .0 09.8 3 0.53 6.42 T 47 8 9 0 3 0 8 4

2 0 1 2 2 1 1 9 2 5 .2E-1 .1 2 4.90 8.75 1.76 T 66 7 9 0 1

5 0 8 4 9 1 3 3 1 C 1 .5E-0 .0 6 .39 .54 .51 T 00 25 03 G13-1 9 6 58

5 5 1 0 4 1 1 3 2 8 3 5 .2E-0 1.23 .0 .70 2.41 T 65 58 4

9 8

1 0 9 1 1 1 2 2 6 4 .5E-0 .0 4 .11 1.16 1.80 T 65 10 3 9 7

2 0 1 8 1 1 2 1 7 5 .2E-0 .0 4 1.36 .45 3.43 T 92 90 3 9 8

2 0 1 8 3 2 2 1 6 6 .5E-0 .0 4 1.43 6.46 1.83 T 51 76 6 9 8

1 0 1 8 1 1 1 2 9 7 .5E-0 .0 5 1.25 .81 3.40 T 56 49 0 9 8

7 0 7 4 8 1 4 3 9 9 .9E-0 .0 5 .33 .05 .33 T 63 35 1 9 5

1 0 1 6 7 8 2 2 1 1 .1E-0 .0 7 0 .96 .12 .69 T 68 60 13 9 5

4 0 1 4 2 1 1 1 3 1 1 .3E-1 .0 2 2.76 8.35 4.21 T 92 10 6 10 0LS-1 0 9 8 7 1 4 0 1 7 1 1 1 2 2 1 .0E-1 .47 .0 3.04 .57 T 35 59 2

0 3

2 0 1 4 2 2 1 9 5 3 .6E-1 .0 3 2.81 0.56 2.43 T 54 6 0 0 9

1 0 1 3 1 1 1 1 2 4 .5E-1 .0 2 1.29 5.79 9.78 T 12 14 9 0 8

3 0 9 3 1 1 1 1 3 6 .4E-1 .0 2 .78 9.85 9.21 T 81 12 7 0 7

1 0 2 6 3 1 1 8 2 7 .8E-1 .1 1 2.40 0.73 6.83 T 41 7 1 0 6

1 0 6 2 1 1 2 1 2 8 .2E-1 .0 1 .06 4.91 1.96 T 00 00 3 0 4

1 0 1 3 2 2 1 1 2 9 .7E-1 .1 1 6.70 9.41 6.08 T 40 40 2 0 2

2 0 1 1 2 1 2 1 1 4 .8E-1 .0 2 0 1.34 6.25 7.59 T 81 15 1 0 8

1 4 1 0 1 2 1 1 9 6 4 2 .6E-1 7.20 .1 8.06 1.57 T 74 8 0

0 2

8 0 1 1 4 2 1 1 1 5 .6E-1 .0 3 4 0.91 4.60 1.47 T 91 35 7 0 8

Note: a, eU (effective uranium) is calcultated as eUppm = [Uppm] +

(0.235*[Thppm]); b, T = Number of terminations identified on crystal; c, L and W =

Length and width of crystal or crystal fragment; d, UC = Estimate uncertainty, i.e., 6% standard deviation of repeat analysis of Durango apatite standards; e, dispersion =

(maximum - minimum)/average, after Brown et al. (2013).

Table 4

Summary of the joint thermochronological data in this study.

K-felds Biotite/

Raw AFT par Muscovite

AHe Age 40Ar/39Ar 40Ar/39Ar

L C P P S C C C ocalit entr Plate late ample losure losure losure y al AZ au au tempe tempe tempe G ( age (°C age age rature rature rature rai Ma (Ma )b (M （ M (°C)a (°C)c (°C)d n # ) ) a) a）

111

± 7

204 T ± 6 1 6 1 he HQ 7 1 64 ± 0–1 0LS-3 transe 5 524 11 20 ct ± 7

67 ±

7 1

± 7

801

± 6

906

± 6

1 1 34 0 ± 8

1 1 21 1 ± 7

T 1 6 1 1 2 3 C he HQ 7 64 ± 0–1 67 50–35 05 20–40 G13-1 transe 5 11 20 ± 1 0 ± 1 0 1 ct

T 212 C 1 6 he HQ ± 7 7 G13-1 21 ± 0–1 transe 4 5 3 7 20 ct 49 ±

59 ±

103

± 6

34 ±

43 ±

4 T C 7 1 6 1 1 1 3 he HQ 7 G13-1 53 ± 12 ± 0–1 54 50–35 70 20–40 transe 5 5 4 5 20 ± 1 0 ± 1 0 ct 6

66 ±

70 ±

91 ±

1 1 13 0 ± 7

16 ±

22 ±

30 ±

3 T 1 2 8 6 he HQ 7 0LS-1 49 ± 5 ± 0–1 transe 5 7 2 7 20 ct 3

67 ±

71 ±

83 ±

92 ±

4 1 1 ± 0 2

6 1 0 ± 2 4

5 1 7 ± 4 3

T C 1 6 1 1 3 he HQ G13-0 18 ± 0–1 50–35 61 20–40 transe 2 7 20 0 ± 1 0 ct

T C 7 6 1 1 1 3 he HQ G12-2 9 ± 0–1 31 50–35 63 00–35 transe 0 4 20 ± 1 0 ± 1 0 ct

Note: a, closure temperature for apatite (U-Th)/He analysis (°C) is from Wolf et al.

(1996); b, apatite PAZ (°C) is from Gleadow et al. (1986); c, closure temperature (°C) for K-feldspar 40Ar/39Ar analysis is from Lovera et al. (1989); d, closure temperature

(°C) for biotite 40Ar/39Ar is from McDougall and Harrison (1999); closure temperature

(°C) for muscovite 40Ar/39Ar is from Hames and Bowring (1994).

Highlights

 First constraint on the exhumation of the Langshan Mountains

 Biotite/muscovite and K-feldspar 40Ar/39Ar, AFT, and AHe analyses

 Individual and joint thermal history modelling

 Impacts of thrusting on denudation

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